As a colorant and a toner to be in a liquid developing agent for electrostatic photography, a printing ink, an ink for an ink-jet system and a coating paint composition, a pigment or dye or a colorant prepared by coating a pigment or dye with a fixing resin should be dispersed in a carrier liquid in a stable manner. Additionally, a printed product, a coated product or an image obtained by the use of the developing agent, ink or coating paint composition should also have a high coloration degree and a high image density and should additionally provide a sharp image. In particular, in a liquid developing agent for electrostatic photography, since the image-forming forming grains of a so-called toner, which are dispersed in a highly-insulating non-aqueous solvent, greatly influence the quality of the image developed with the agent, toner grains in particular should have good dispersibility.
Two methods--a dry developing method and a liquid developing method--in general are approaches used forming a visual image from an electrostatic latent image as formed in an electrostatic photographic process. The liquid developing method is superior to the dry developing method, since minute toner grains can be used and fine images or halftone images can be faithfully reproduced.
In general, a liquid developing agent for electrostatic photography comprises a colorant such as various pigments or dyes, with carbon black being a typical example, a coating agent which adsorbs onto or coats the colorant for adjusting the charge of the colorant or for imparting a fixing property to the colorant, a dispersing agent for dispersing the toner grains, a charge-adjusting agent to adjust the polarity and the amount of charge of the toner grains, and a highly electrically-insulating non-aqueous solvent carrier liquid having a dielectric constant of 3 or less and a volume resistivity of 10.sup.9 .OMEGA.cm or more. The agent does not need to contain a pigment or dye, when it is applied to a printing plate where the toner image should have only an ink-adhering property, when it is used in preparing a printing plate using an etching liquid, the agent also does not need to contain a pigment or dye and only needs to have resist properties. A liquid developing agent, a printing ink and a coating paint composition are generally prepared using the following method. More specifically, a pigment or dye and a fixing resin, or a pigment or dye coated with a fixing resin is converted directly into a powder of the desired size or in a dry system. Thereafter it is dispersed in a dispersing agent-containing a high-electrical resistance non-aqueous solvent using a wet-dispersing machine such as a ball mill, a paint shaker or a sand mill, to obtain a thick dispersion. The thus prepared thick dispersion is used directly or after dilution with a solvent. In preparing a liquid developing agent, the thick dispersion is added to a charge-adjusting agent-containing carrier liquid to obtain a liquid developing agent having a positive or negative polarity.
Alternatively, a different method can be used in which monomers capable of forming a polymer which is insoluble in a carrier liquid are polymerized in the carrier liquid in the presence of a polymer which is soluble in the carrier liquid to form resin grains (dispersion polymerization), and the grains are dyed with a dye, or they are added to a charge-adjusting agent-containing carrier liquid along with a separately dispersed pigment to prepare a liquid developing agent, an ink or a coating paint composition. Accordingly, a mechanical dispersing means or a granulating means for dispersion polymerization or the like is necessary in order to prepare toner grains of a liquid developing agent, or an ink or a coating paint composition. Additionally, this is conducted in a carrier liquid, and the formed dispersion is stored before use in the carrier liquid or diluted with a dispersing liquid or carrier liquid. Accordingly, the dispersion being stored is influenced by the ambient temperature and humidity during transportation or storage before use. This results in a variation in the characteristics of the dispersion, such as grain size or viscosity thereof, as compared with the characteristics of the corresponding fresh dispersion immediately after preparation. As discussed above, in the case of a liquid developing agent, a charge-adjusting agent is added to the toner grains present in the liquid developing agent in order that the positive or negative polarity of the grains and the charged amount thereof are held constant. Additionally, the toner further contains a fixing resin for the purpose of providing fixing properties thereto, in addition to a pigment or dye. Therefore, the charge-adjusting agent and the fixing resin often separate from the pigment or dye (de-coating) as a result of variations in the ambient temperature and humidity conditions during transportation or storage or with the lapse of time on storage. This results in variations in the charged amount or a coagulation of the toner grains. As a result, the characteristics of the stored liquid developing agent vary which adversely influences the quality of the images formed with the agent.
In order to overcome the problems, for example, search for a more stable charge-adjusting agent, development of a better combination of pigment and coating agent resin, and search for a coating agent resin which scarcely undergoes de-coating are being conducted. In addition, investigation of better dispersing means and search for a better dispersing agent are also being conducted to overcome the time-dependent variation in the dispersing properties of the developing agent. However, since the above-mentioned phenomena are intrinsic problems in a liquid dispersion system it is difficult to completely solve these problems.
The best method of solving the problems is to prepare a liquid developing agent (toner) just before its use and the fresh agent is not stored but is used immediately. One example of a method is to add a toner for supply to a developing agent being used in depending on the amount of the toner used. However, since the toner supplied in this method is a high-density dispersion, the toner of itself varies with the lapse of time during storage thereof. Accordingly, this method can not be used to solve these problems.
JP-A-1-113766 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a toner for supply which is in the form of a solid tablet. This is prepared by tableting toner grains with a binder resin which is soluble in a carrier liquid. In the illustrated method, a colorant or a flashed colorant and a resin are first dispersed in a liquid carrier with a dispersing agent to obtain a thick dispersion, and this is filtered by forced filtration, then dried and powdered with a mixer to obtain a powder. Next, the resulting powder is tableted with a binder which is soluble in the carrier liquid. Suitable binders which can be used are paraffin wax, higher fatty acids, higher alcohols and higher fatty acid esters. However, the method is extremely complicated, and the forced filtration of the thick dispersion is, in particular, not practical. Additionally, drying of the filtered dispersion residue induces re-coagulation of the toner grains, whereby a redispersion of the tableted toner is difficult.
JP-A-63-194270 illustrates a method where a high polymer substance having a melting point in the vicinity of room temperature is added to a commercial liquid developing agent to gel and solidify the liquid developing agent, and the solidified agent is heated to liquify it just before use. In this method, the agent may be either a solid or a liquid by repeatedly cooling and heating the same. The solid state as referred to therein means a gel state which contains a liquid component. Therefore, the developing agent gel becomes liquid when the ambient temperature rises. In practical use of this agent, this agent is often inconvenient to use since it becomes liquid as the ambient temperature rises even though it should be solid.
In view of the above, a solid self-dispersing colorant which is capable of dispersion by itself to form a stable dispersion only when it is added to a carrier liquid, as well as a liquid developing agent for electrostatic photography which contains such a colorant, and a toner for supply to a liquid developing agent and a toner kit also containing such a colorant have not been developed at present.